Optical amplifiers are essential components employed in optical communication networks for boosting optical signals weakened by propagation losses, splitting and filtering. A number of types of optical amplifiers have been developed. One class of optical amplifiers is rare-earth doped optical amplifiers, which use rare-earth ions as the active element. The ions are doped in the fiber core and pumped optically to provide gain. The silica core serves as the host medium for the ions. While many different rare-earth ions such as neodymium, praseodymium, ytterbium etc. can be used to provide gain in different portions of the spectrum, erbium-doped optical amplifiers have proven to be particularly attractive because they are operable in the spectral region where optical loss in the silica core is minimal. Also, the erbium-doped optical amplifier is particularly useful because of its ability to amplify multiple wavelength channels without crosstalk penalty, even when operating deep in gain compression.
There are two primary types of rare-earth doped optical amplifiers, fiber optical amplifiers and planar waveguide optical amplifiers. Fiber amplifiers are optical glass fibers having cores doped with the optically active element. Waveguide amplifiers are planar waveguides fabricated on suitable substrates such as silicon. The planar waveguide amplifiers include cores doped with optically active elements.
Fiber amplifiers are often favored over waveguide amplifiers in most applications because waveguide amplifiers are typically much shorter, requiring a correspondingly higher level of doping to achieve the necessary gain of fiber amplifiers. Higher levels of doping adversely affect the efficiency of the amplifier. However, waveguide amplifiers are easier to manufacture than fiber amplifiers because large numbers of them can be fabricated on a single wafer and integrated on the same chip with other components making the whole unit less expensive. Therefore, in applications requiring relatively small components, device integration and ease of packaging, it is advantageous to use waveguide amplifiers.
One problem with waveguide amplifiers in comparison to fiber waveguides is their high scattering losses, which may be three orders of magnitude greater than in a fiber and which limits the overall gain that can be achieved.